In general, known as a ballpoint pen are a gravity type (free atmospheric type) ballpoint pen comprising a ballpoint pen tip, an ink reservoir and an ink filled thereinto and a so-called pressurized type ballpoint pen in which pressurized gas such as nitrogen gas is charged into an ink reservoir filled with an ink and a closed vessel containing it.
The above pressurized type ballpoint pen has the advantages that upward writing is possible and that an ink having a specific function can be used, but an ink reservoir charged with an ink is always pressurized, so that a small amount of the ink is apt to leak from a pen tip.
In order to prevent the above matter, an apparent viscosity of an ink has to be raised, and this brings about the problems that a write feeling becomes heavier and it takes time to charge an ink reservoir with an ink and that the ink is liable to be stuck onto the inside of the ink reservoir, that is, a so-called clear drain property is inferior.
On the other hand, it is described in Japanese Patent Publication No. 48565/1972 that the main point is to prepare a writing instrument in which an ink or a fluid is not allowed to flow out in the vicinity of a ball of the ballpoint pen tip in the form of a pressurized writing or marking instrument, and disclosed therein is a ballpoint pen writing instrument characterized by that a writing fluid for the above writing instrument comprises at least one solvent, one colorant and at least one polymer which is soluble in the above solvent and which can provide the above fluid with a viscoelasticity, a strong coagulating property, a strong sticking property, a fluid resistance and a film-forming property together with the other components for the writing fluid, wherein the above writing fluid has a viscosity of 100000 cps or more (measured at 25° C. and 0.3 rpm using a fifth cylindrical spindle in a Brookfield LMT viscometer), and a cross-linking carboxypolymethylene type polymer or polyvinylpyrrolidone type polymer or both of them is used as the above polymer.
Further, a thixotropic ink for a pressurized ballpoint pen containing higher fatty acid soap is disclosed in Japanese Patent Application Laid-Open No. 096336/1975, and the viscosities of 600000 cps and 800000 cps are given in the examples.
Further, an ink composition having metallic gloss which is set to a viscosity of 50000 cps or more is disclosed in Japanese Patent Application Laid-Open No. 186574/1985. Also, an ink for a pressurized ballpoint pen used for a plotter is disclosed in Japanese Patent Application Laid-Open No. 7775/1987, and it is described that the viscosity thereof falls preferably in a range of 40000 to 500000 cps.
However, any ink compositions for pressurizing disclosed in the respective official gazettes described above have a high viscosity, and there still remain the problems described above, that is, the problems that an ink leaks from a pen tip is small amount; a write feeling becomes heavier; it takes time to charge an ink reservoir with an ink; an ink is liable to be stuck onto the inside of an ink reservoir, and a so-called clear drain property is inferior, and so on.
On the other hand, an oil-based ink for a ballpoint pen containing silica having a primary average particle diameter of 7 to 40 nm is disclosed in Japanese Patent Application Laid-Open No. 195365/1998 which is a prior art applied by the present inventors.
In the above prior art, however, it is described in the specification that “a pen body rises in a temperature when exposed to direct sunlight or an ink leaks from a pen tip in a certain case when left standing downward for long time”, and it is a technique related to a gravity type (free atmospheric type) ballpoint pen. More specifically, it is described that a content of silica is 0.01 to 2.0% by weight, preferably 0.03 to 1.0% by weight, and it is 0.3% by weight, 1.0% by weight, 0.05% by weight and 0.5% by weight respectively in order in Examples 1 to 4. A supplementary examination carried out by applying ballpoint pens specifically described in Example 2 (1.0% by weight) and Example 4 (0.5% by weight) to pressurized ballpoint pens under the same conditions as in examples described later has made it clear to bring about the problem that an ink leaks from a pen tip. Further, observation of the above oil-based ink under an optical microscope has shown that coagulated matters containing silica are not found and that the ink stays in a homogeneous state. Accordingly, the oil-based ink described in the above official gazette is different from the ink of the present invention in uses, recognition of the problems and ink physical properties.
In light of the problems on the conventional techniques described above, the present invention intends to solve them, and an object thereof is to provide an ink composition for a pressurized ballpoint pen and a pressurized ballpoint pen, wherein an ink is prevented from leaking from a pen tip; a write feeling is excellent; time for charging an ink reservoir with the ink can be shortened; and a clear drain property is excellent as well.